Astronomy

Do boulders erode differently on asteroids than on the Moon?

Do boulders erode differently on asteroids than on the Moon?


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If I understand it correctly, boulders on the Moon are only found near fresh craters, because micrometeorites erode them over time. Asteroids are believed to have formed sometimes even earlier than the Moon, but some images show asteroids covered by boulders. And NASA is planning the ARM mission to go pick up a boulder from an asteroid.

Are boulders more frequent on asteroids than on the Moon, and if so, by what kind of mechanism? For example, it is not primarily micrometeorites, but temperature changes that erodes them on the Moon. Or don't microgravity objects attract as many micrometeorites as the Moon does?

Intact boulders on asteroid 25143 Itokawa:

Eroded boulders on the Moon:


The moon does attract more particles. But more importantly, those particles hit the moon with velocities of the order of the Moon's escape velocity, 2.4 km/s. That's a very powerful sandblaster…


Do boulders erode differently on asteroids than on the Moon? - Astronomy

How do you know what an asteroid is made out of, and what is the classification system for asteroids?

There are many ways to tell what asteroids are made of. One way is to send a spacecraft there, for example NASA's NEAR spacecraft that orbited Eros for a year. While in orbit, the spacecraft used an infrared camera and spectrometer and an x-ray/gamma ray spectrometer to look at the composition of the asteroid.

However, we can't send spacecraft to every asteroid, and most asteroid compositions are determined using infrared spectroscopy from ground based telescopes. In the infrared, different minerals absorb different wavelengths of light. By looking at the infrared spectral absorptions, and comparing them to spectra of minerals measured on Earth, it is possible to identify the composition. This is still a difficult process, though, because asteroids are faint, and so it can be difficult to get a good enough detection to be sure about the spectrum. Usually you need to observe the object for a significant fraction of its rotation period which means that you get one spectrum for the entire object you can't see compositional differences between different regions on the asteroid. Also, asteroids are combinations of many minerals, and so astronomers argue over what combination of minerals can form a particular asteroid spectrum. Sometimes it looks like several mineral combinations could give you similar spectra, so it can be hard to tell which one is correct.

Another way to determine the composition is to use radar. With planetary radar you send out a radio signal to the asteroid and look at what is reflected back. The radio waves react differently to different materials, for example metals look quite different from rock. This is a relatively new technique, since only in the past couple decades have we had the technology to look at lots of small and faraway objects with radar. Therefore, there aren't as many asteroids that have been classified based on radar observations.

Asteroids are classified using a lettering system which, in my opinion, is one of the more non-intuitive classification systems in astronomy. There are usually 14 classifications (A,B,C,D,E,F,G,M,P,Q,R,S,T,and V), but some scientists don't believe that some of the classifications should be distinct and some of the classifications seem to contain many more types. The asteroids are placed into a letter group based on their spectral characteristics, not based on their "real-life" characteristics, but some of the letters correspond to familiar things. For example, S type asteroids are "stony" and M type are probably metallic. Just as an example, the description for the "A" classification might be "extremely reddish shortward of 0.7 microns strong absorbtion feature longward of 0.7 microns. " (quoted from Tholen and Barucci, 1989).

To add to the confusion, meteorites, which are pieces of planets or asteroids that have fallen to Earth, are classified in a similar but separate manner. This is because we can measure the meteorites in a lab, so it's much easier to tell what they are made of. Right now, people are trying to connect the meteorite classes with the asteroid classes, but it's hard because we don't get the same information from telescopic spectroscopy that we get from a lab on Earth. So it's possible that one type of meteorite could come from asteroids in multiple classes, or several types of meteorite could come from a single asteroid class.


What astronomers mean by 'rubble pile asteroids'

[Note: I realized a while back that astronomers use jargon that may be confusing to normal people, so I thought it would be fun to write the occasional piece defining such terms. That way y'all can understand it better, and I can link to it in later articles without having to explain myself over again. It's not just an excuse to post super-cool images of the asteroid Bennu. Not "just."]

When I was a kid, the general idea about asteroids — at least among the public — is that they were big, monolithic chunks of rock and/or metal orbiting the Sun. If you happened to come upon one in your spaceship (which happened by accident all the time thanks to movies and TV shows) it would be vaguely spherical, and covered in smooth lumps and eroded craters.

That turns out not to be the case. Maybe really big ones look a little bit like that (I'm thinking Vesta and Ceres, but really planetary scientists consider them protoplanets and not asteroids), but many, perhaps most of the smaller ones are nothing at all like that.

Take a typical chunk of asteroid in the main belt between Mars and Jupiter, an object that's a kilometer across — pretty small. Maybe it formed that size billions of years ago with everything else in the solar system, or perhaps a very large asteroid was hit by a not-quite-so-large asteroid, creating debris, including our little friend.

But what happens next? Well, it's not alone. Over the course of millions and even billions of years as it orbits the Sun, other asteroids will occasionally smack into it. In general these aren't high-speed impacts like we think of them that form big craters, like the ones we see on the Moon or Mercury. The majority of these impacts come from rocks moving on similar orbits, so the impacts are much lower in velocity, maybe a kilometer or two per second instead of ten or twenty.

That's still an energetic event! It's not catastrophic, but it is, literally, world-shattering. The shock wave from the impact travels through the asteroid, cracking it, creating fissures that can run deep below the surface. And this happens again and again, cracking and recracking the asteroid. What does it look like after, say, a billion or three years?

It looks like this. Like Bennu.

The asteroid Bennu, a “rubble pile”, seen by the OSIRIS-REx spacecraft from a distance of 13 km. Note the huge boulder on the lower right. Credit: NASA/Goddard/University of Arizona

Bennu is just such a 500-meter wide asteroid * , the target of NASA’s OSIRIS-REx mission. It’s not a main belt asteroid it orbits the Sun much closer to Earth, and it gets so close to us it’s considered to be a Potentially Hazardous Asteroid because it has a small chance of impacting us some day in the far future. However, we think many main belt asteroids are very similar to it.

Right away you can see it’s weird. The double-pyramid base-to-base shape I’ve explained before it’s due to low gravity, rapid spin, and weak structure (as I’ll get to in a sec). But even from a distance you can see the surface isn’t at all like depicted in movies. On a large scale it’s smooth, with no craters visible at all! That’s weird.

But then when you get close up it’s a whole lot weirder. Look at the surface:

A long rock (center), possibly the remnants of a shattered, even bigger boulder, lies on the surface of the asteroid Bennu. It’s nearly 19 meters long, almost the length of tennis court. Credit: NASA/Goddard/University of Arizona

Yeah, see what I mean? That's not some smooth, mildly lumpy landscape. It’s sharp, jagged, and covered in rocks and boulders! Those are fragments of the asteroid itself, possibly created in those slow-speed impacts. They lie jumbled everywhere on the surface… and maybe Bennu is like this all the way down. Cracked, fractured, made up of nothing but boulders of all different sizes all the way through.

There’s another possibility. There’s a sweet spot for impacts, one big enough to crack it all the way through and even kick a lot of that debris away, but not quite energetic enough to totally disrupt the asteroid (that is, break it into bits that all fly away forever). In that case, the debris may stay close by, and eventually re-aggregate, coming back together. Most likely there would be a whole bunch of these kinds of impacts, closer to the lower-energy end, such that over time, again and again, the asteroid is ruptured and jostled and then recoalesces.

It would be a gigantic collection of individual rocks held together by their own (if very weak) gravity. What would you call such a thing?

Astronomers call it a rubble pile. That's pretty apt.

One image in particular from OSIRIS-REx fascinated me:

Rocks of different sizes on Bennu appear to be sorted by size, possibly due to an event that shook the surface, sending them rolling downhill. Credit: NASA/Goddard/University of Arizona

You can see how there are big rocks at the bottom of the frame, and they get progressively smaller toward the top. Geologists call this kind of pattern sorting, where some mechanism sorts the rocks by size. Sometimes that happens in floods, where small rocks are carried farther than big ones. Obviously there’s no flowing water on Bennu, but another possible mechanism could be from small impacts jolting the asteroid, causing asteroidquakes. A big pile of jumbled rubble could then collapse, and it’s possible the different sized rocks would travel different distances as they slowly rolled downhill.

The largest boulder on the surface of Bennu, called Boulder No. 1 (or, unofficially, BenBen). It’s 22 meters high, as tall as a 6 story building. Credit: NASA/Goddard/University of Arizona

I also wonder what the interior is like. There’s a process called granular segregation, sometimes called the Brazil Nut Effect: In a jar of mixed nuts, weirdly Brazil nuts, which are generally the largest, are found on top. Why? Simply put, it’s because there are gaps between the nuts, and when the can gets jostled smaller things (fragments, dust, and so on) can settle down toward the bottom, so the bigger nuts get pushed up.

We don’t see any dust on the surface of Bennu. I wonder if it's all in the interior, settled down between the cracks, while other boulders rise to the surface? It seems likely, but I’m speculating.

A different angle on BenBen, the largest boulder on Bennu. Credit: NASA/Goddard/University of Arizona

All this is more than just a really cool thing to know. It affects a lot about the asteroid, including how well it can take a punch. One reason you don’t see many (or any!) craters on the surface is that the asteroid is so porous that the energy from a small impact just gets absorbed into the asteroid it displaces rocks but doesn’t create a sharp crater. Also, the very small impacts I just mentioned could, over time, cause the rocks to settle, filling in some of the gaps. Any crater won’t last terribly long on a cosmic scale.

Big rocks on the surface of Bennu form a spine-like ridge, like a line of shark’s teeth, from the equator toward the south. Credit: NASA/Goddard/University of Arizona

And this knowledge is critical if we happen to see an asteroid this size on a collision trajectory with Earth. If we try to push it out of the way by hitting it with a spacecraft —a kinetic impactor — the asteroid will respond differently depending on its composition. We need to know what will happen when we whack it, so understanding how such an object behaves could literally be the difference between life and death.

The largest boulder on Bennu, called BenBen, casts a shadow across an assortment of rocks on the surface. Note all the different reflectivities of the rocks some are quite dark while others reflect more light. Credit: NASA/Goddard/University of Arizona

So, while the term "rubble pile" may sound a little whimsical, rest assured that astronomers take it very seriously. We want to know what these things are made of out of scientific curiosity, to be sure, but there may come a day when that knowledge will fill a far more pressing role.

More Bad Astronomy

* CORRECTION (May 20, 2019): I originally wrote that Bennu is 1 km wide, but it is 500 meters wide (I had confused it with Ryugu, a very similar asteroid being visited by Hayabusa2). My thanks to Erin Morton, Communications Lead for the OSIRIS-REx mission, for pointing this out to me!


Space Rocks Hit the Moon Often

By: Camille M. Carlisle October 13, 2016 1

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Observations from NASA’s Lunar Reconnaissance Orbiter reveal how often the Moon is being pockmarked by meteoroids.

Lots of space rocks hit Earth, the Moon, and the other bodies in the solar system. On Earth erosion erases craters fairly fast, but on the Moon — which has no atmosphere or liquid water — craters can stay around much longer.

Using images from NASA’s Lunar Reconnaissance Orbiter, Emerson Speyerer (Arizona State University) and colleagues have now taken a close look at just how often Earth’s natural satellite is being pummeled. The team scoured more than 14,000 before-and-after pairs of images from the spacecraft’s narrow-angle camera, separated by anywhere from 176 to 1,241 days, and searched for changes between the two shots. (Don’t worry, they didn’t do that by hand — they used software.)

The researchers found 222 new craters, ranging from 43 meters (140 feet) down to less than 10 m (33 ft) across, spread fairly uniformly across the lunar surface. Here's one of them:

This before-and-after sequence shows two images from the Lunar Reconnaissance Orbiter's narrow-angle camera: the first from October 25, 2012, and the second from April 21, 2013. Between that time, a 12-meter-wide crater appeared.
NASA / GSFC / Arizona State University

Extrapolating, it means that roughly 180 craters at least 10 m wide or larger form on the Moon each year. That’s 33% more than the rate predicted by a standard model.

At face value that sounds like a big difference, but in fact there is still some overlap: there’s a 16% chance of the two predictions agreeing. That’s actually pretty cool. The old model that Speyerer’s team is comparing their results to, called the Neukum production function, comes from work done in the 1970s and is based on direct observations of overlapping craters on surfaces that scientists have dependable dates for, thanks to samples nabbed by Apollo astronauts. From these measurements, scientists estimated how often craters of various sizes should form over million-year times scales, explains Caleb Fassett (Mount Holyoke College), who works on crater statistics but wasn’t involved with the present study. The fact that those million-year-scale estimates agree within 16% of the new ones, which are on a year-level time scale, is “remarkable,” he says.

The new rate is also still basically in agreement with the number of impacts we detect in Earth's atmosphere, says Peter Brown (University of Western Ontario, Canada). Given the uncertainties in all the various way we measure and estimate impact rates, a 33% uptick is nothing notable, he says — it'd really need to be more than a factor of 10 off before the difference raised questions. Nevertheless, the new LRO data are a unique and fantastic addition to the other ways scientists study how common small asteroids are, he says. (A good rule of thumb is that a 0.5-meter-wide object will create a 10-meter-wide lunar impact.)

What difference there is does have implications. Impacts churn the uppermost centimeters of the lunar surface, called regolith, kind of like someone turning over soil for planting. Speyerer’s team found evidence for this mixing in LRO’s images, in the form of dark and light splotches. The scientists estimate that the insistent little impacts “garden” the Moon’s loose surface material so much so that the top 2 cm of regolith is reworked in about 81,000 years, or more than 100 times faster than expected.

The results appear in the October 13th Nature. Below, you can watch a brief video about how the analysis worked.


How scientists are 'looking' inside asteroids

The shape of asteroids such as 243 Ida can reveal information about what they’re made of, which can, in turn, tell us more about the formation of the solar system. Credit: NASA/JPL/USGS

Asteroids can pose a threat to life on Earth but are also a valuable source of resources to make fuel or water to aid deep space exploration. Devoid of geological and atmospheric processes, these space rocks provide a window onto the evolution of the solar system. But to really understand their secrets, scientists must know what's inside them.

Only four spacecraft have ever landed on an asteroid—most recently in October 2020—but none has peered inside one. Yet understanding the internal structures of these cosmic rocks is crucial for answering key questions about, for example, the origins of our own planet.

"Asteroids are the only objects in our solar system that are more or less unchanged since the very beginning of the solar system's formation," said Dr. Fabio Ferrari, who studies asteroid dynamics at the University of Bern, Switzerland. "If we know what's inside asteroids, we can understand a lot about how planets formed, how everything that we have in our solar system has formed and might evolve in the future."

Then are also more practical reasons for knowing what's inside an asteroid, such as mining for materials to facilitate human exploration of other celestial bodies, but also defending against an Earth-bound rock.

NASA's upcoming Double Asteroid Redirection Test (DART) mission, expected to launch later this year, will crash into the 160m in diameter asteroid moon Dimorphos in 2022, with the aim of changing its orbit. The experiment will demonstrate for the first time whether humans can deflect a potentially dangerous asteroid.

But scientists have only rough ideas about how Dimorphos will respond to the impact as they know very little about both this asteroid moon, and its parent asteroid, Didymos.

To better address such questions, scientists are investigating how to remotely tell what's inside an asteroid and discern its type.

During the fourth ever landing on an asteroid, Bennu was mapped thanks to a mosaic of images collected by NASA’s OSIRIS-REx spacecraft. Peering inside an asteroid is the next crucial step. Credit: NASA/Goddard/University of Arizona

There are many types of asteroids. Some are solid blocks of rock, rugged and sturdy, others are conglomerates of pebbles, boulders and sand, products of many orbital collisions, held together only by the power of gravity. There are also rare metallic asteroids, heavy and dense.

"To deflect the denser monolithic asteroids, you would need a bigger spacecraft, you would need to travel faster," said Dr. Hannah Susorney, a research fellow in planetary science at the University of Bristol, the UK. "The asteroids that are just bags of material—we call them rubble piles—can, on the other hand, blow apart into thousands of pieces. Those pieces could by themselves become dangerous."

Dr. Susorney is exploring what surface features of an asteroid can reveal about the structure of its interior as part of a project called EROS.

This information could be useful for future space mining companies who would want to know as much as possible about a promising asteroid before investing into a costly prospecting mission as well as knowing more about potential threats.

"There are thousands of near-Earth asteroids, those whose trajectories could one day intersect with that of the Earth," she said. "We have only visited a handful of them. We know close to nothing about the vast majority."

Dr. Susorney is trying to create detailed topography models of two of the most well-studied asteroids—Itokawa (the target of the 2005 Japanese Hayabusa 1 mission) and Eros (mapped in detail by the NEAR Shoemaker space probe in the late 1990s).

"The surface topography can actually tell us a lot," Dr. Susorney said. "If you have a rubble pile asteroid, such as Itokawa, which is essentially just a bag of fluff, you cannot expect very steep slopes there. Sand cannot be held up into an infinite slope unless it's supported. A solid cliff can. The rocky monolithic asteroids, such as Eros, do tend to have much more pronounced topographical features, much deeper and steeper craters."

Coloured topographical maps from Dr Susorney show Eros (left), a rocky monolithic asteroid, as having steeper craters than Itokawa (right), a rubble pile asteroid. Credit: Hannah Susorney

Susorney wants to take the high-resolution models derived from spacecraft data and find parameters in them that could then be used in the much lower resolution asteroid shape models created from ground-based radar observations.

"The difference in the resolution is quite substantial," she admits. "Tens to hundreds of metres in the high-res spacecraft models and kilometres from ground-based radar measurements. But we have found that, for example, the slope distribution gives us a hint. How much of the asteroid is flat and how much is steep?"

Dr. Ferrari is working with the team preparing the DART mission. As part of a project called GRAINS, he developed a tool that enables modelling of the interior of Dimorphos, the impact target, as well as other rubble pile asteroids.

"We expect that Dimorphos is a rubble pile because we think that it formed from matter ejected by the main asteroid, Didymos, when it was spinning very fast," Dr. Ferrari said. "This ejected matter then re-accreted and formed the moon. But we have no observations of its interior."

An aerospace engineer by education, Dr. Ferrari borrowed a solution for the asteroid problem from the engineering world, from a discipline called granular dynamics.

"On Earth, this technique can be used to study problems such as sand piling or various industrial processes involving small particles," Dr. Ferrari said. "It's a numerical tool that allows us to model the interaction between the different particles (components) - in our case, the various boulders and pebbles inside the asteroid."

The researchers are modelling various shapes and sizes, various compositions of the boulders and pebbles, the gravitational interactions and the friction between them. They can run thousands of such simulations and then compare them with surface data about known asteroids to understand rubble pile asteroids' behaviour and make-up.

The solar system’s asteroid belt contains C-type asteroids, which likely consist of clay and silicate rocks, M-type, which are composed mainly of metallic iron, and S-type, which are formed of silicate materials and nickel-iron. Credit: Horizon

"We can look at the external shape, study various features on the surface, and compare that with our simulations," Dr. Ferrari said. "For example, some asteroids have a prominent equatorial bulge," he says, referring to the thickening around the equator that can appear as a result of the asteroid spinning.

In the simulations, the bulge might appear more prominent for some internal structures than others.

For the first time, Dr. Ferrari added, the tool can work with non-spherical elements, which considerably improves accuracy.

"Spheres behave very differently from angular objects," he said.

The model suggests that in the case of Dimorphos, the DART impact will create a crater and throw up a lot of material from the asteroid's surface. But there are still many questions, particularly the size of the crater, according to Dr. Ferrari.

"The crater might be as small as ten metres but also as wide as a hundred metres, taking up half the size of the asteroid. We don't really know," said Dr. Ferrari. "Rubble piles are tricky. Because they are so loose, they might as well just absorb the impact."

No matter what happens on Dimorphos, the experiment will provide a treasure trove of data for refining future simulations and models. We can see whether the asteroid behaves as we expected and learn how to make more accurate predictions for future missions that lives on Earth may very well depend on.


Thermal data can shed new light on lunar craters

To determine the ages of lunar impact craters, Mazrouei and her colleagues made use of temperature data from the Diviner instrument on NASA’s Lunar Reconnaissance Orbiter, which has been in orbit around the moon for the past 10 years.

Large rocks on the moon’s surface hold on to their heat far longer than the fine lunar sand, or regolith. While the regolith cools down fairly quickly during the cold lunar night, the largest rocks on the moon’s surface are able to stay warm.

Mazrouei and colleagues realized that they could use this temperature-dependence to their advantage. Recent impact craters on the moon are associated with large pieces of rock ejecta, while the oldest craters have been worn down by small impacts over time and are composed almost entirely of lunar regolith.

“As craters get older,” Mazrouei explained in an article for The Conversation, “they become less rocky.”

By examining the temperatures of different craters, the team was able to determine whether they were made up of large rocks or small bits of regolith. This in turn allowed them to estimate the craters’ ages.


Recommended Reading

The Best-Ever Photos of an Asteroid’s Rugged Terrain

The Worst Day in Earth’s History Contains an Ominous Warning

The Asteroid That Smote the Dinosaurs Burned the Birds Out of Trees


Do boulders erode differently on asteroids than on the Moon? - Astronomy

Meteoroids, Meteors, and Meteorites

No spacecraft has yet landed on an asteroid and no samples of the Martian surface have been brought back to Earth, but we have pieces of these bodies nonetheless -- the rocks are called meteorites, rocks that have fallen from the sky. The reality of rocks falling from the sky wasn't always accepted, but today we know that rocks really do fall from space. Some are large enough to do significant damage, blasting huge craters upon impact. Despite the forces of erosion, more than 100 impact craters can still be seen on Earth.

Rocks from space continue to rain down on Earth, and observers find a few meteorites every day. Meteorites are often blasted apart in their fiery descent through the atmosphere, scattering fragments over an area several kilometers across. A direct hit on the head would be fatal, but there are no reliable accounts of human deaths from meteorites. However, meteorites have injured at least one human (by a ricochet, not a direct hit), damaged houses and cars, and killed animals. Most meteorites fall into the oceans, which cover three-quarters of the Earth's surface.

Before interplanetary debris enters the atmosphere, it is called a meteoroid . It is a small fragment of matter traveling through interplanetary space -- destined eventually to strike another object. Of course comets, asteroids, and planets also travel through the void of interplanetary space, but a meteoroid differs from these chiefly in its size, no more than a few meters (feet) in diameter, usually much less.

While in the atmosphere, the falling body is called a meteor . As a meteor moves swiftly through the atmosphere, it is heated rapidly by friction, leaving behind it a bright trail. The spectacle is often called a "shooting star".

If a meteor survives its plunge through the atmosphere and strikes the Earth's surface, it is then called a meteorite . Thus, rocks fallen from space are meteorites only if they are located on the Earth's surface or for that matter on the surface of any planet or moon.

Most meteorites are difficult to distinguish from terrestrial rocks without a detailed scientific analysis, but there are a few clues that help. Meteorites are usually covered with a dark, pitted crust resulting from their fiery passage through the atmosphere. Some can be readily distinguished from terrestrial rocks by their high metal content. The ultimate judge of extraterrestrial origin is laboratory analysis of the rock's composition. The presence of certain rare elements such as iridium is one major indicator of extraterrestrial origin since nearly all of Earth's iridium sank to the core long ago and hence is absent from surface rocks.

In terms of physical and chemical composition, meteorites fall into three broad classes: irons, stones, and stony-iron. The irons , which are generally about 90% iron and 9 percent nickel, with a trace of other elements, are the most commonly found. The stones are composed of low-density silicate materials similar to the Earth's crustal rocks. When examined closely under a microscope, many stones are seen to contain silicate spheres, called chondrules . embedded in a smooth surface. These meteorites are therefore known as chondrites Finally, stony-iron meteorites represent a crossbreed between the iron and the stones and commonly exhibit small stone pieces set in iron.

One of the most curious kinds of chondrite is the carbonaceous chondrite . The chondrules in such meteorites are embedded in material that contains a large fraction of carbon compared with other stony chondrites -- typically about 2 percent carbon compared to the total mass. Their carbon content gives these meteorites a dark appearance. Carbonaceous chondrites also contain significant fractions of water, usually about 10 percent by mass.

Refer to your reading assignment for more on the properties of meteorites.

The vast majority of meteorites appear to be composed of material from the solar nebula, and radioactive dating shows that they were formed at the same time as the solar system itself -- about 4.6 billion years ago. These primitive meteorites are our best source of information about the conditions in the solar nebula.

A much smaller group of meteorites appears to have undergone substantial change since the formation of the solar system, and radioactive dating shows some of these meteorites to be younger than the primitive meteorites. Astronomers call this group processed meteorites because they apparently were once part of a larger object that "processed" the original material of the solar nebula into another form.

The primitive meteorites may be either carbon-poor stones or carbonaceous chondrites. Carbon compounds condense only at the relative low temperatures that were found in the solar nebula beyond 3 AU from the Sun. Thus, astronomers conclude that the carbonaceous chondrites are small rocks (asteroids only a few meters across or less) that came from the outer region of the asteroid belt or in rare instances from dust tails of comets.

The carbon-poor stones presumably formed closer to the Sun. Thus, they appear to be the remains of material from the surface of tiny asteroids in the inner part of the asteroid belt.

The processed meteorites tell a more complex story. Their compositions are similar to the cores, mantles, or crusts of terrestrial-like bodies. Thus, they must be fragments of the interior of worlds that underwent differentiation just like the terrestrial planets. That is, they are from worlds that must have been heated to high enough temperatures to melt inside, allowing metals to sink to the center and rocks to rise to the surface. It is believed that large asteroids went through a period of active volcanism shortly after they formed. Thus, processed meteorites resembling lava may have been chipped off the surface of a large asteroid by relatively small collisions. Those with corelike or mantlelike compositions (iron or stony-irons) may, on the otherhand, be fragments of asteroids that completely shattered in collisions. Thus, processed meteorites offer astronomers an opportunity to study a large dissected-asteroid.

If basaltic fragments can be chipped off the surface of an asteroid, is it possible that fragments can be chipped off a larger object such as a moon or planet? In fact, a few processed meteorites have been found that don't appear to match the compositions of asteroids but instead appear to match the composition of the Moon and Mars. Astronomers are now confident that they came from violent impacts on the Moon and Mars, which sent ejected surface material into interplanetary space. The analysis of these lunar meteorites and Martian meteorites is providing new insight into the conditions on the Moon and Mars. In at least one case, a Martian meteorite may be offering astronomers clues about whether life once existed on Mars.

Refer to your reading assignment for more on the origin of meteorites.


For this topic, study the true and false, fill in the blanks self-test, and review questions at the end of the Chapter(s) of your reading assignment. In addition, learn the key words and answer all questions that follow:

Key Terms (refer to your text for some these terms)

meteoroid
meteor
meteorite
irons
stones
stony-irons
iridium
chondrule
chondrite
carbonaceous chondrite
primitive meteorite
lunar meteorite
Martian meteorite

Review Questions (refer to your text to answer some of these questions)

1. How do meteoroids differ from asteroids and comets?
2. What is a meteor and how is it related to a "shooting star?"
3. Explain how meteors differ from meteorites.
4. Based on composition, what are the three types of meteorites?
5. What is a chondrite?
6. What are primitive meteorites?
7. What are processed meteorites?
8. What is the origin of carbonaceous chondrites?
9. What is the origin of carbon-free stones?
10. What do processed-meteorites tell astronomers about asteroids?

Advanced Questions (refer to your text to answer these questions)

1. How do astronomers know that primitive meteorites came from the asteroid belt?
2. Explain how the existence of processed meteorites tells astronomers that some asteroids once had active volcanism.
3. What evidence suggests that some processed meteorites came from the Moon and Mars?

As an option, you may post your questions to a message board. Each tutor and faculty member has a message board. Answers to your questions will be posted within 24 hours. You can also review questions and answers that have been sent to the message board during the past seven days.

Do you need more help with this topic?

Check the Astronomy On-Line Resource Center .


What are the chances of hitting earth?

Concerned yet? It’s scary to think just how often these events occur. In fact, there is a 1 in 20,000 chance of being killed by an impact in your life time (though this number is hotly debated, and some put it around 50,000 to 200,000…). To put that in perspective, it’s about the same chance you have of being killed in an airplane, though we tend to fear the latter a lot more than the former.

Fortunately, NASA in 1998 established the Near-Earth Objet Program Office at JPL to essentially track the skies and detect oncoming hazardous asteroids that could threaten earth. But, they only track the large ones, so the little but deadly ones can still sneak by theoretically… if they aren’t small enough to dissolve in the atmosphere that is.

It is important to note that along with identifying threat, the Near-Earth Object Program allows scientist to study these passing asteroid for important research, and possibly future resource (some day… they need geologists to study space rocks right)?


How Scientists Are ‘Looking’ Inside Asteroids? (Planetary Science)

Asteroids can pose a threat to life on Earth but are also a valuable source of resources to make fuel or water to aid deep space exploration. Devoid of geological and atmospheric processes, these space rocks provide a window onto the evolution of the solar system. But to really understand their secrets, scientists must know what’s inside them.

Only four spacecraft have ever landed on an asteroid – most recently in October 2020 – but none has peered inside one. Yet understanding the internal structures of these cosmic rocks is crucial for answering key questions about, for example, the origins of our own planet.

‘Asteroids are the only objects in our solar system that are more or less unchanged since the very beginning of the solar system’s formation,’ said Dr Fabio Ferrari, who studies asteroid dynamics at the University of Bern, Switzerland. ‘If we know what’s inside asteroids, we can understand a lot about how planets formed, how everything that we have in our solar system has formed and might evolve in the future.’

Then are also more practical reasons for knowing what’s inside an asteroid, such as mining for materials to facilitate human exploration of other celestial bodies, but also defending against an Earth-bound rock.

NASA’s upcoming Double Asteroid Redirection Test (DART) mission, expected to launch later this year, will crash into the 160m in diameter asteroid moon Dimorphos in 2022, with the aim of changing its orbit. The experiment will demonstrate for the first time whether humans can deflect a potentially dangerous asteroid.

But scientists have only rough ideas about how Dimorphos will respond to the impact as they know very little about both this asteroid moon, and its parent asteroid, Didymos.

To better address such questions, scientists are investigating how to remotely tell what’s inside an asteroid and discern its type.

There are many types of asteroids. Some are solid blocks of rock, rugged and sturdy, others are conglomerates of pebbles, boulders and sand, products of many orbital collisions, held together only by the power of gravity. There are also rare metallic asteroids, heavy and dense.

‘To deflect the denser monolithic asteroids, you would need a bigger spacecraft, you would need to travel faster,’ said Dr Hannah Susorney, a research fellow in planetary science at the University of Bristol, the UK. ‘The asteroids that are just bags of material – we call them rubble piles – can, on the other hand, blow apart into thousands of pieces. Those pieces could by themselves become dangerous.’

Dr Susorney is exploring what surface features of an asteroid can reveal about the structure of its interior as part of a project called EROS.

This information could be useful for future space mining companies who would want to know as much as possible about a promising asteroid before investing into a costly prospecting mission as well as knowing more about potential threats.

‘There are thousands of near-Earth asteroids, those whose trajectories could one day intersect with that of the Earth,’ she said. ‘We have only visited a handful of them. We know close to nothing about the vast majority.’

During the fourth ever landing on an asteroid, Bennu was mapped thanks to a mosaic of images collected by NASA’s OSIRIS-REx spacecraft. Peering inside an asteroid is the next crucial step. Image credit – NASA/Goddard/University of Arizona

Dr Susorney is trying to create detailed topography models of two of the most well-studied asteroids – Itokawa (the target of the 2005 Japanese Hayabusa 1 mission) and Eros (mapped in detail by the NEAR Shoemaker space probe in the late 1990s).

‘The surface topography can actually tell us a lot,’ Dr Susorney said. ‘If you have a rubble pile asteroid, such as Itokawa, which is essentially just a bag of fluff, you cannot expect very steep slopes there. Sand cannot be held up into an infinite slope unless it’s supported. A solid cliff can. The rocky monolithic asteroids, such as Eros, do tend to have much more pronounced topographical features, much deeper and steeper craters.’

Susorney wants to take the high-resolution models derived from spacecraft data and find parameters in them that could then be used in the much lower resolution asteroid shape models created from ground-based radar observations.

‘The difference in the resolution is quite substantial,’ she admits. ‘Tens to hundreds of metres in the high-res spacecraft models and kilometres from ground-based radar measurements. But we have found that, for example, the slope distribution gives us a hint. How much of the asteroid is flat and how much is steep?’

Coloured topographical maps from Dr Susorney show Eros (left), a rocky monolithic asteroid, as having steeper craters than Itokawa (right), a rubble pile asteroid. Image credit – Hannah Susorney

Dr Ferrari is working with the team preparing the DART mission. As part of a project called GRAINS, he developed a tool that enables modelling of the interior of Dimorphos, the impact target, as well as other rubble pile asteroids.

‘We expect that Dimorphos is a rubble pile because we think that it formed from matter ejected by the main asteroid, Didymos, when it was spinning very fast,’ Dr Ferrari said. ‘This ejected matter then re-accreted and formed the moon. But we have no observations of its interior.’

An aerospace engineer by education, Dr Ferrari borrowed a solution for the asteroid problem from the engineering world, from a discipline called granular dynamics.

‘On Earth, this technique can be used to study problems such as sand piling or various industrial processes involving small particles,’ Dr Ferrari said. ‘It’s a numerical tool that allows us to model the interaction between the different particles (components) – in our case, the various boulders and pebbles inside the asteroid.’

‘Asteroids are the only objects in our solar system that are more or less unchanged since the very beginning of the solar system’s formation.’

— Dr Fabio Ferrari, University of Bern, Switzerland

Rubble pile

The researchers are modelling various shapes and sizes, various compositions of the boulders and pebbles, the gravitational interactions and the friction between them. They can run thousands of such simulations and then compare them with surface data about known asteroids to understand rubble pile asteroids’ behaviour and make-up.

‘We can look at the external shape, study various features on the surface, and compare that with our simulations,’ Dr Ferrari said. ‘For example, some asteroids have a prominent equatorial bulge,’ he says, referring to the thickening around the equator that can appear as a result of the asteroid spinning.

In the simulations, the bulge might appear more prominent for some internal structures than others.

For the first time, Dr Ferrari added, the tool can work with non-spherical elements, which considerably improves accuracy.

‘Spheres behave very differently from angular objects,’ he said.

The model suggests that in the case of Dimorphos, the DART impact will create a crater and throw up a lot of material from the asteroid’s surface. But there are still many questions, particularly the size of the crater, according to Dr Ferrari.

‘The crater might be as small as ten metres but also as wide as a hundred metres, taking up half the size of the asteroid. We don’t really know,’ said Dr Ferrari. ‘Rubble piles are tricky. Because they are so loose, they might as well just absorb the impact.’

No matter what happens on Dimorphos, the experiment will provide a treasure trove of data for refining future simulations and models. We can see whether the asteroid behaves as we expected and learn how to make more accurate predictions for future missions that lives on Earth may very well depend on.

The solar system’s asteroid belt contains C-type asteroids, which likely consist of clay and silicate rocks, M-type, which are composed mainly of metallic iron, and S-type, which are formed of silicate materials and nickel-iron. Image credit – Horizon

The research in this article was funded by the EU.

Featured image:The shape of asteroids such as 243 Ida can reveal information about what they’re made of, which can, in turn, tell us more about the formation of the solar system. Image credit – NASA/JPL/USGS


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